The present invention relates to a tubing shoe for use in well bores as are typically utilised in oil and gas production.
After boring or drilling a region of an oil or gas well a “string” of tools and/or tubing is typically run into the well bore. As the string is run it can meet obstructions as it travels through the well bore. These obstructions may be ledges that form from well material during boring, formation wash-outs, or debris formed by unstable sections of the well bore wall collapsing. Bridges of shale and clay stone can also be formed. Such obstructions can result in the string jamming in the well bore.
To prevent or minimise the effect of these obstructions, a guide shoe is conventionally mounted on the lower end of the string.
For example, after boring a region of an oil or gas well, it is normal to run tubing or casing into the well bore to act as a lining. The casing is typically run into the well bore from the surface and the length of casing is often referred to as a “casing string”. The lining of the well bore can then be strengthened by introducing cement between the external surface of the casing and the internal surface of the well bore. As the casing is run there is a risk of the casing string jamming as it meets obstructions in the well bore. To prevent or minimise the effect of the obstructions, a guide shoe, referred to as a reamer shoe, is conventionally mounted on the lower end of the casing string.
A typical reamer shoe has two features; a nose portion designed to guide the casing through the centre of the wellbore, so reducing the risk of the casing string jamming against the bore wall, and a reaming portion around the body of the shoe which removes any irregularities or obstructions from the wall of the bore, and thereby ease the passage of the casing string. When the casing is successfully positioned and set in place, the nose portion may be drilled out to leave a throughbore for the passage of tools to drill and case the next section of the well bore. The dual purpose of the shoe requires that the material of the nose cone is soft, and therefore easily able to be drilled out and the material of the reaming portion must be hard, so that it can successfully remove obstructions on the wall of the bore. Ideally, the shoes are constructed of two materials; a body comprising the reamer is made of a hard material, such as steel, while the nose portion is made of a soft material, such as aluminium. The shoes are typically a two-part construction, with the nose portion screwed into an annular sleeve that includes the reamers.
Reamer shoes generally may be used in two modes; the casing string and reamer shoe may be rotated and advanced in the manner of a drilling operation, alternatively the casing string and the reamer shoe may be reciprocated to provide a rasping action against partial obstruction in the well bore. In general, the reciprocating mode would be preferable when the threaded casing connections are considered too weak to support the rotational torsion required to turn and ream away at obstructions. In order that a single design of reamer shoe may be conveniently used in either manner, certain combinations of features have been brought together in a single unit. It will be seen that although these combinations improve performance in certain aspects, they compromise performance in other aspects.
Certain reamer shoes incorporate helical reaming members giving full circumferential coverage to assist in rasping the entire bore hole wall when operated in the reciprocating mode. An example of such a shoe is that disclosed in U.S. Pat. No. 6,401,820. This feature may be seen to be detrimental in certain circumstances by reference to the manner in which casing joints are mated together. Casing joints are invariably threaded and screwed together prior to running into the well bore. It will be appreciated that upon engaging an obstruction and attempting to overcome it by reciprocation, a helical reaming member will inevitably impart a rotational action to the casing string as it slides over and past the obstruction. Depending on whether the helical reaming member is clockwise or anti-clockwise, the reamer shoe may impart a tightening or untightening torsion to the threaded connections higher up in the casing string. As has been stated, threaded casing connections may be relatively weak and could be damage if over-tightened. Conversely, if rotated in the opposite direction, the connections may be loosened. Either outcome is undesirable and could result in serious consequences for the well bore construction operation.
A further undesirable consequence of anti-clockwise helical reaming members may be apparent when this style of reamer shoe is used in the normal clockwise rotational mode. The rotating helical members impart a restraining influence on the flow of well bore fluid and in particular on the solid components entrained in the fluid, generated by the reaming process. The result is a gradual increase in the concentration of solid material ahead of the reaming elements that can pack-off the reaming area, rendering it ineffective. In order to clean the reaming members it may be necessary to pick-up the reamer shoe and circulate fluid at a high rate, if this is unsuccessful, then the reamer shoe along with the entire casing string would have to be removed from the well bore. It will be appreciated that this is a highly undesirable operation.
Another design of reamer shoe uses multiple diamond-shaped reaming members to overcome the negative aspects of the helical reaming design. US 2003/0075364 provides an example of diamond-shaped reaming members. A feature of this design is that each reaming element has a leading edge. It will be apparent to those skilled in the art, that the leading edge of each element is a potential site for hanging-up whilst tripping into the well bore. Hanging-up is a phenomenon where tools that ideally can be run into a well bore with a smooth and uninterrupted action, may intermittently come to a halt when sudden changes in a section of the tool string and of the well bore come into contact. Hanging-up is at best an inconvenience, at worst, it can result in the entire casing string being pulled from the well to investigate the cause of the problem.
Yet another design of reamer shoe uses a reaming structure that converges towards the forward end of the nose of the reamer shoe. This design is illustrated in U.S. Pat. No. 6,062,326. One undesirable consequence of this design is that relatively large pieces of well bore formation may pass by the reaming members without being ground-up. If these pieces exceed a certain size, they may not be carried back to the surface by the flow of well bore fluid. In this event, they can fall back to the upper end of the reamer shoe and collect there. There are certain common circumstances where this may be an undesirable outcome. Firstly, after reaming to the bottom of the well bore, it is normal practice to cement at least the lower section of the casing string, including the reamer shoe itself, in place. In order that a good strong cement bond is made it is important that the well bore fluid along with contaminants such as cuttings are circulated out before the cement is put in place. With large pieces of well bore formation collecting above the reamer shoe, this may not be possible. The outcome could be a contaminated and therefore weak cement bond. Secondly, if it were necessary to reciprocate the casing string when the upper part of the reamer shoe had a collection of cuttings above it, it could be seen that on the upstroke the cuttings would become jammed between the reamer shoe and the hole-wall. In the worst circumstances, it may not be possible to free the reamer shoe and the casing would have to be set in the position that it became jammed.
It is an object of the present invention to provide a shoe that overcomes these and other limitations of existing shoes.